A 9000 kg boxcar traveling at 17.0 m/s strikes a second boxcar at rest. The two stick together and move off with a speed of 6.5 m/s. What is the mass of the second car?

Answer 1

If the two cars stuck together, it is an inelastic collision, and energy is not conserved during the collision.

However, momentum is conserved, and since there is only one common final velocity, v, only the momentum equation is enough to solve for the unknown mass.
The equation of conservation of momentum:
m1 v1 + m2 v2 = (m1+m2)v ..... (1)
m1=9000 kg
m2=unknown
v1=17 m/s
v2=0
v=6.5 m/s

Solve for m2.